Process of producing alkali-metal carbonates



June 23, 1925. 1,542,911

F. w. SPERR, JR., ET AL PROCESS OF PRODUCING ALKALI METAL CARBONATES Original Filed April 18. 1921 E l Mu# 50u/naiv 7, a f Y 1 f AIR '00H57 alia/Rafa uws/1 suur/wv l `7 a f O t Puna/"E @ms AER/afmamwm sns scm/asm (1L JZ j b@ganan/WIW," TANK G57 U Mn M Patented I June 23,y 1925i i UNITED STATESl 1,542,971 PATENT OFFICE.

y FREDERICK W. SPERR, JR., AND DAVID L. IIACOBSON, F PITTSBURGH, PENNSYL- VANIA, ASSIGNORS TO THE KOPPERS COMPANY, 0F- PITTSBURGH, PENNSYLVANIA.,

.A CQBIORATION OF PENNSYLVANIA.

PROCESS 0F PRODUCING ALKALI-MEITAL .CARBONATES appncation ined Apri; 1s, 1921,"se1-ia1Nopisaaos. Renewed August 22, 1924.

To aZZ whom 'it may concern Be it known that we, FREDERICK W. SPERR, Jr., and DAVID L. JAooBsoN, citizens of the United States, residing in Pittsburgh, in 5 the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Processes of VProducing Alkali-Metal Carbonates, of which the following is a specification.

This invention relates to the removal of hydrogen-sulphide from gases by bringing such gases into contact with a liquid me dium which may be continuously regenerated by aeration; the invention has for its object to provide for the regeneration of the secondary products of aeration that are inert with respect to the hydrogen sulphide and that cannot, without such regeneration,

play any part in its removal. In-the coding application of Frederick W. Spern gigi and David L. Jacobson, Serial No.

458265, April 4, 1921, there is described a pross for the manufacture of alkali-metal ,y thiosulphates in which gas, containing -hy- 5 drogen sulphide, is' brought into contact with an alkaline solution which is continuously circulated and aerated. The present invention contemplates the application of such a process to the substantially complete removal of hydrogeny sulphide from the gas, combined with a substantially complete conservation of the alkali, by providing for the regeneration of the thiosulphate formed in r the purification rocess, such regeneration converting the t iosulphate princi ally to carbonate, which may be employed or making up the alkaline solution used for treating the gas.

The such as cake oven gas, water gas, an other gases, to which this process 1s commonly applicable, usually contain C502 which is a useful nt in the process. If, however, the gases o not originell contain CO2, it may either be added arti cially or else gases containing G02, such as waste l stack gas, may be used in the treatment of the solution to produce the requisite amount of sodium bicarbonate.

In addition to the general objects recited above, the invention has for further objects such other improvements or advantages in operation and result as are found to obtain in the apparatus and process hereinafter described or claimed.

gen sulphide and carbon dioxide wlth a so? lutionof an alkaline purifying-agent such as sodi-um carbonate, the 'following reactions occur:

When the resulting solution containing sodium bicarbonate and sodium hydrosulphide together with unchanged sodium carbonate is aerated, part of the sodium hydrosulphide is changed to sodium thiosulphate. The remainder together with part of the sodium bicarbonate is decomposed in accordance with the following reactions:

2NaHCO3=Na2CO3+CO2+H2O QNaHS-l-COZ-l-HZO:NaZCOS-l-QHZS LNaHCOB-i-NaHSzNaZCOa-l-HZS According to the invention, the alkali thiosulphate, in the present instance the so` `dium thiosulphate, is converted principally into sodium carbonate (NazCOB). Some sodium sulphide (NazS) may also be formed,

but since this, in solution, is readily hydrolyzed to sodium hydrosul hide (NaHS) and sodium hydroxide (NaH), it may be returned together with the sodium carbonate to the gas unification process.

A speci c application of the invention will now be described. 4

Circulating tanks A and B contain the alkaline solution used in the process. The process may, for example, be started .with a solution containing from 3 to 5% sodium carbonate, although other alkaline comouncls,- such for exam 1e as potassium caronate may be used. girculatin tank B is connected with the gas scrubber bymeans of the pipeline D which has a short branch line communicating with storage tank E and controlled b f valve I". The scrubber may be any well 'iown type of gas scrubber, for

example, a so-called benzol scrubber filled with Wooden hurdles. The solution is pumped from B b means of the pump G over the scrubber C and, containing the hydrogen sulphide absorbed from the gas, passes out through the pipe J into the circulating tank A. The gas enters the scrubber through the pipe H and passes out thro-ugh the pipe I substantially free from its`original hydrogen sulphide content. The solution is pumped from the tank A by pump K through pipe line L over the aerating tower M which may be an apparatus similar to the scrubber C, or may be a natural draft cooling tower in which the solution is simply aerated by being brought into contact with the atmosphere under natural draft. In the type of apparatus in which air isblown by means of the fan N. The exit air containing H25 passes out through the pipe O and may be disposed of in any desired Way. The aerated solution, containing sodium thiosulphate, runs throurrh the pipe P into the circulating tank B. preferably maintained in continuous operation. When the sodium thiosulphate has accumulated to a desired percentage the valve F is opened and a portion of the solution is transferred into the storage tank E. The remaining solution in the circulating system is made up to its original volume by adding the alkaline solution obtained from the subsequent treatment of the sodium thiosulphate to the contents of the tank A.

It is preferable before transferring this solution from the circulating system into the storage tank E to discontinue the addition of fresh sodium carbonate until the percentage of sodium carbonate in the circulating solution is reduced to about 1 or 2%. This Will save the separation and handling of a considerable amount of sodiumcarbonate in the subsequent treatment of the solubicarbonate is thereby partly decomposed to carbonate, and from 80 to 90% of the total carbonate is present in solid form in the hot solution. This carbonate is separated by hot filtration or centrifuging in centrifuge S. The centrifuged solution runs into crystallizing tank T wherein it is allowed to tThe system as shown is cool. Most of the sodium thiosulphate crystallizes out and the mixture of crystals and mother liquor is returned to contrifuge S. The separated mother liquor runs into tank In case it contains any considerable amount of sodium thiocyanate, it may be treated for the recovery or utilization of this material and fresh sodium carbonate should be added to the circulating system to replace the equivalent of thiocyanate removed. Otherwise, the mother liquor itself or any thiosulphate subsequently recovered from it may be returned to storage tank E or to a fresh batch of liquor treated in evaporator R. The sodium thiosulphate crystals from theV centrifuge S are mixed with somematerial high in carbon, such as coal, and present case, there is employed a scrubberl with limestone, the three materials being finely divided and intimately admixed. 'Suitable proportions are one part sodium thiosulphate, one and one-half parts limestone, and one part coal, although the proportions may be varied as found desirable. The mixture is introduced into a' suitable furnace, such as a muiiie or reverberatory furnace'V and heated to a high temperature, such as 1000o C. During the heating the mixture is preferably stirred in any convenient manner. The fused mass is then Withdrawn from the furnace and leached with Water in the leaching apparatus W. The resulting solution contains principally sodium 4carbonate although other soluble alkalies may be present. This solution is suitable for use in the gas purification system. The solution is separated from any carbonaceous material by filtration and returned to the circulating system Where it is placed in tank A in sufficient amounts to maintain the strength of the circulating solution at the desired percentage of alkalinity. The sodium carbonate recovered from the liquor treated in evaporator R is also added to this solution, so that all of the alkali sodium compounds originally used are recovered and returned to the circulating system, with the exception of any that may have been lost mechanically or through lthe formation of thiocyanate.

An alternate method of converting the sodium thiosulphate to carbonate is to heat the sodium thiosulphate by itself in a suitable furnace, such as a mufile or reverberatory furnace, to a high temperature. The fused mass is then Withdrawn from the furnace, mixed with coal and limestone and again heated to a high temperature in the furnace, the process being completed as described above. This alternative method is the specific subject of the co-pending application of the same applicants tiled May 19, 1925, Serial No. 31457.

The sodium thiosulphate decomposes atv a high temperature and the decomposed products react With the coal and limestone,

forming principally sodium carbonate although other soluble alkalies such as sodium alkali-metal carbonate solution which passes cyclically back and forth through the gas washing stage and through an aeration stage, the latter involving the formation of alkali-metal `thiosulphates in the solution,

the improvement characterized by recovering such thiosulphates from the solution, heating the -recovered thiosulphates admixed with carbonaceous material and limestone to convert them to "carbonates and restoring the thus recovered carbonates to the gas Washing solution and thereby substantially maintaining the solution at Ithe alkalinity required for the continuous 'washing of the gases; substantially as specified.

2. In a cyclic process for the purification of gases in which the gas is Washed by a sodium carbonate solution which passes cyclically back and forth through the gas washing stage and through an aeratlon stage, the latter involving the formation of sodium thiosulphates in the solution, the improvement characterized by recovering such thiosulphates from the solution, heating the recovered thiosulphates admixed with carbonaceous material and limestone to convert them to ycarbonates and restoring the thus recovered carbonates to the gas Washing solution and thereby substantially maintaining the solution at the alkalinity required for the continuous Washing of gases; substantially as specified.

3. In a cyclic gas-purification process in which the gas is washed by an alkali-metal carbonate solution. which asses cyclically back and forth through 't e gas washing stage and through an aeration stage, the lat- -ter involving the formation of alkali-metal thiosulphates in the solution, the improvement characterized by recovering such thio-A sulphates from the solution and converting them to carbonates, and restoring the thus recovered carbonates to the gas washing solution and thereby substantially maintaining the solution at the alkalimty required for the continuous washing lof gases; substantially as-specified.

4'.' The process of'producing alkali-metal carbonates which consists in mixing an alkali thiosulphate with carbonaceous material and limestone, subjecting the admixture to heating vto convert the thiosulphate to n carbonate, and separating'the alkali-metal carbonate from the residual carbonaceous material;l substantially as specified.

5. The process of `producing alkali-metal carbonates which consists in mixing an alkali thiosulphate with coal and limestone, subjecting the mixture to heating to convert the thiosulphate to carbonate, and separatino the alkali-metal carbonate from the residual coal; substantially as specified.

6. In the recovery of alkali as alkali-carbonates from gas purification liquors containing alkali-metal thiosulphates, the improvement consistin in recoveringl such thiosulphates from said liquors in solid form and heating the recovered thiosulphates admixed with carbonaceous material `and limestone to convert them to valkali-carbonates In testimony whereof we have hereunto set our hands. f 

